This invention generally relates to goal functions or visual function diagnostic metrics, and in particular provides methods, devices, and systems for mitigating or treating vision conditions such as presbyopia, often by determining a compound modulation transfer function.
Presbyopia normally develops as a person ages, and is associated with a natural progressive loss of accommodation, sometimes referred to as “old sight.” The presbyopic eye often loses the ability to rapidly and easily refocus on objects at varying distances. There may also be a loss in the ability to focus on objects at near distances. Although the condition progresses over the lifetime of an individual, the effects of presbyopia usually become noticeable after the age of 45 years. By the age of 65 years, the crystalline lens has often lost almost all elastic properties and has only limited ability to change shape. Residual accommodation refers to the amount of accommodation that remains in the eye. A lower degree of residual accommodation contributes to more severe presbyopia, whereas a higher amount of residual accommodation correlates with less severe presbyopia.
Known methods and devices for treating presbyopia seek to provide vision approaching that of an emmetropic eye. In an emmetropic eye, both distant objects and near objects can be seen due to the accommodation properties of the eye. To address the vision problems associated with presbyopia, reading glasses have traditionally been used by individuals to add plus power diopter to the eye, thus allowing the eye to focus on near objects and maintain a clear image. This approach is similar to that of treating hyperopia, or farsightedness.
Presbyopia has also been treated with bi-focal eyeglasses, where one portion of the lens is corrected for distance vision, and another portion of the lens is corrected for near vision. When peering down through the bifocals, the individual looks through the portion of the lens corrected for near vision. When viewing distant objects, the individual looks higher, through the portion of the bi-focals corrected for distance vision. Thus with little or no accommodation, the individual can see both far and near objects.
Contact lenses and intra-ocular lenses (IOLs) have also been used to treat presbyopia. One approach is to provide the individual with monovision, where one eye (usually the primary eye) is corrected for distance-vision, while the other eye is corrected for near-vision. Unfortunately, with monovision the individual may not clearly see objects that are intermediately positioned because the object is out-of-focus for both eyes. Also, an individual may have trouble seeing with only one eye, or may be unable to tolerate an imbalance between their eyes. In addition to monovision, other approaches include bilateral correction with either bi-focal or multi-focal lenses. In the case of bi-focal lenses, the lens is made so that both a distant point and a near point can be focused. In the multi-focal case, there exist many focal points between near targets and far targets.
Surgical treatments have also been proposed for presbyopia. Anterior sclerostomy involves a surgical incision into the sclera that enlarges the ciliary space and facilitates movement of the lens. Also, scleral expansion bands (SEBs) have been suggested for increasing the ciliary space. Problems remain with such techniques, however, such as inconsistent and unpredictable outcomes.
In the field of refractive surgery, certain ablation profiles have been suggested to treat the condition, often with the goal of increasing the range of focus of the eye, as opposed to restoring accommodation in the patient's eye. Many of these ablation profiles can provide a single excellent focus of the eye, yet they do not provide an increased depth of focus such that optimal distance acuity, optimal near acuity, and acceptable intermediate acuity occur simultaneously. Shapes have been proposed for providing enhanced distance and near vision, yet current approaches do not provide ideal results for all patients.
To evaluate the effectiveness of a refractive correction, such as with a spectacle lens, contact lens, intra-ocular lens, or laser refractive surgery procedure, it may be desirable to consider a merit function, or gauge of optical quality, that can determine such effectiveness. Gauges of optical quality are discussed in copending patent application Nos. 60/431,634, filed Dec. 6, 2002, 60/468,303, filed May 5, 2003, and 10/738,358 filed Dec. 5, 2003, the disclosures of which are hereby incorporated by reference. Merit functions may be used in evaluating post-corrective measurements, and in predicting the effect or outcome of a proposed corrective procedure. While the merit function may be objective, it may also desirable that the merit function have a good correlation with subjective test results such as visual acuity, contrast acuity, and the like. The following optical metrics can be or have been used as possible optical metrics or merit functions: high order (HO) root mean square (RMS) error; Strehl ratio; modulation transfer function (MTF) at specific spatial frequencies; volume under MTF surface up to a certain spatial frequency; compound MTF; encircled energy; and wavefront refractions. Other goal functions or visual function diagnostic metrics are available for characterizing lenses and other optical systems, including visual acuity such as log MAR, refractive error such as sphere and cylinder, and contrast sensitivity (CS). However, many of the currently used goal functions are difficult and cumbersome to implement with current clinical methods, and are insufficient in utilizing currently available clinical data and in providing guidance to the administration and diagnosis of reported visual difficulties.
In light of the above, it would be desirable to have improved methods, devices, and systems for treatment and/or mitigation of optical defects, based on improved goal functions such as a compound modulation transfer function. The goal functions should be easily implemented with existing clinical data, and with clinical data that is currently being generated by present measurement techniques. Optionally, it would be desirable to have improved methods, devices, and systems for treatment and/or mitigation of presbyopia and other optical defects. It may be desirable to provide improved prescriptions in the form of practical customized or optimized prescription shapes for treating or mitigating vision conditions such as presbyopia in a particular patient.